1. Field of the Invention
This invention relates to a steel cord, particularly a flattened steel cord used as a reinforcement in rubber articles such as pneumatic tires and industrial belts and the like and a pneumatic tire using such a cord.
Further, the invention relates to a process for producing the above steel cord and a tubular-type twisting machine used therefor.
2. Description of Related Art
Various structures are known in the steel cord reinforcing a pneumatic tire as a typical example of a rubber article. In recent years, it is proposed to flatten the steel cord in order to improve various properties of the steel cord for use in the tire. That is, there is proposed a steel cord comprising a core formed by bundling three or more filaments without twisting each other and at least one sheath formed by winding a plurality of filaments around the core. This type of the cord has advantages that the anisotropy of the bending rigidity is large and the tensile rigidity is high as compared with a steel cord having a core formed by twisting a plurality of filaments or a core comprised of two untwisted filaments. And also, it is not required to twist the filaments as the core as compared with a steel cord having a core formed by twisting a plurality of filaments, so that it is advantageously possible to produce the steel cord at a few number of steps.
For example, JP-A-63-176702 discloses a steel cord comprising a core comprised of three filaments arranged in parallel to each other and a sheath comprised of plural filaments surrounding therearound.
In such a cord, however, the core filaments arranged in parallel extend straightforward in the longitudinal direction thereof, so that when tensile load is applied to the cord, the core filaments preferentially bear such a load and hence the bearing efficiency of tensile load as a whole of the cord lowers and the durability of the cord is poor. And also, the tensile rigidity is high on one hand and the elongation is low on the other hand, so that the cord has a disadvantage that the absorption energy through the elongation deformation is small.
On the other hand, JP-A-9-158065 discloses a steel cord having a core comprised of three filaments arranged without twisting and such a cross sectional shape of the cord that an elliptical shape and an approximately true circular shape are mixed in the longitudinal direction of the cord. In this cord, remarkably different cross sections are existent in the longitudinal direction of the cord, so that the bending deformation is not uniform in the longitudinal direction of the cord and the durability to bending is degraded.
It is, therefore, an object of the invention to provide a steel cord, particularly a flattened steel cord comprising a core formed by arranging plural untwisted filaments side by side and having an excellent tensile rigidity without damaging the bending anisotropy as well as a pneumatic tire having an excellent durability.
It is another object of the invention to provide a process for producing the above steel cord and a tubular-type twisting machine used therefor.
According to a first aspect of the invention, there is the provision of in a steel cord comprising a core formed by bundling three or more filaments side by side without twisting and a sheath of at least one layer comprised of plural filaments wound around the core, an improvement wherein an arrangement of the filaments constituting the core at a section perpendicular to a longitudinal direction of the core differs between at least a part in the longitudinal direction of the core and the other part thereof, and all filaments constituting the core in all section parts are arranged in a rectangle having a long side of dxc3x97(n+1) and a short side of dxc3x97(1+1/2xc2xd) when a diameter of the filament is d and the number of the filaments in the core is n.
In a preferable embodiment of the first aspect, all filaments constituting the core are arranged in a rectangle having a long side of dxc3x97(n+0.5) and a short side of dxc3x97(1+1/2).
In another preferable embodiment of the first aspect, the arrangement of the filaments constituting the core has different parts within one winding pitch of the sheath.
In the other preferable embodiment of the first aspect, a difference between one winding pitch of the sheath and a straight-extended length of each filament constituting the core existent in one winding pitch is is 0.9-1.1 times a stretchable amount of the sheath in an axial direction of the cord within one winding pitch of the sheath.
In a further preferable embodiment of the first aspect, the number of filaments in the core is three or four.
In a still further preferable embodiment of the first aspect, the filaments in the core are closed to each other.
In a further preferable embodiment of the first aspect, the sheath is one layer.
In a still further preferable embodiment of the first aspect, a direction of maximum diameter in the core is substantially the same at any cross section in the longitudinal direction of the core.
In the other preferable embodiment of the first aspect, the cord is flat and a direction of the long size in the cord is substantially coincident with the direction of the maximum diameter in the core.
According to a second aspect of the invention, there is the provision of in a pneumatic tire comprising a carcass as a main skeleton toroidally extending between a pair of bead portions and a belt comprised of plural layers arranged outside the carcass in a radial direction thereof, an improvement wherein steel cords as defined above are applied to at least one layer of the belt so as to arrange a direction of a maximum diameter in the core along a widthwise direction of the belt.
According to a third aspect of the invention, there is the provision of a process for producing a steel cord comprising a core formed by bundling plural filaments without twisting and a sheath formed by winding plural filaments around the cord through a tubular-type twisting machine, which comprises introducing the filaments constituting the core through a position located inside a barrel of the tubular-type twisting machine and separated from the barrel into an assemble portion integrated with the filaments constituting the sheath.
In a preferable embodiment of the third aspect, the filaments constituting the sheath are wound around the filaments constituting the core between a pair of rollers arranged in the assemble portion.
In another preferable embodiment of the third aspect, the filaments constituting the sheath are wound around the filaments constituting the core in a twisting die having a flat-shaped hole at its section arranged in the assemble portion.
In the other preferable embodiment of the third aspect, the steel cord after twisting is corrected by correcting rollers.
According to a fourth aspect of the invention, there is the provision of a tubular-type twisting machine comprising a rotating barrel and an assemble portion for integrating plural filaments at an outside of the rotating barrel and on a rotating axis of the barrel, wherein all of bobbins each wound with the filament are arranged at an inside of the rotating barrel.
In a preferable embodiment of the fourth aspect, the bobbins each wound with the filament constituting a core of a steel cord are arranged at the side of the assemble portion as compared with the bobbins each wound with the filament constituting a sheath of the steel cord.
In another preferable embodiment of the fourth aspect, a pair of rollers are arranged in the assemble portion.
In the other preferable embodiment of the fourth aspect, a twisting die having a flat-shaped hole at its section is arranged in the assemble portion.